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Abstract

New Re–Os geochronology for the Ballachulish Slate Formation of the Dalradian Supergroup, Scotland yields a depositional age of 659.6 ± 9.6 Ma. This age represents the first successful application of the Re–Os system to rocks that have extremely low Re and Os abundances (<1 ppb and <50 ppt, respectively). The Re–Os age represents a maximum age for the glaciogenic Port Askaig Formation and refutes previous chemostratigraphic and lithostratigraphic studies which correlated the Port Askaig Formation with a series of middle Cryogenian (ca. 715 Ma) glacials. Additionally, the Re–Os age strongly suggests that the Port Askaig Formation may be correlative with the 650 Ma end-Sturtian glaciations of Australia. As a consequence, the correlation of the Ballachulish Limestone Formation with the ca. 800 Ma Bitter Springs anomaly is not tenable. Initial Os isotope data from the Ballachulish Slate Formation coupled with data from Australia reveals a radiogenic 187Os/188Os isotope composition (0.8–1.0) for seawater during the Neoproterozoic, which is similar to that of modern seawater (1.06).
We also report a young, highly imprecise Re–Os age (310 ± 110 Ma) for the Early Cambrian Leny Limestone Formation which is constrained biostratigraphically by a polymerid and miomerid trilobite fauna. We suggest, based on the mineralogy of the Leny Limestone, (kaolinite, muscovite and a serpentine group mineral, berthierine), that the Re–Os systematics have been disturbed by post-depositional fluid flow associated with Palaeozoic igneous intrusions. However, it is evident from the Ballachulish Slate Formation results that anhydrous metamorphism does not disturb the Re–Os geochronometer.